The optimal growth conditions for differentiation of the selected neuroblastoma clonal cell line, SY5Y. which will be used for the development of an in vitro assay for the neurovirulence of live poliovirus vaccine preparations, have been determined (selection of agents, their dosage and treatment times). Differentiation was defined by morphological examination; immunohistochemical studies have been initiated using antibodies to neuronal cell markers of differentiation.